The integration of renewable energy into the main power supply network challenges the energy grid, since it was designed for central power production. Electricity generated by renewable energy sources has an unlimited precedence for power supply to the electricity network to support the integration in the energy system and increase its share of the electricity production. Energy production from renewable energy sources is difficult to forecast and depends on weather conditions such as wind speed and solar radiation. To handle this fluctuating production, renewable energy sources have to be curtailed, fossil fueled power plants providing base load need to become more flexible or energy prices reduce strongly due to the high supply. In addition, the location of production of renewable energy such as onshore and offshore wind does not coincide with the region of high power consumption.
Therefore, energy storages play an important role in the improvement of the stability of power supply networks.
Sensible thermal storages are state of the art for storing fluctuating energy from renewable sources. Electrical excess energy from the main supply grid is transformed into thermal energy and is stored in some storage material. In times with no or low occurrence of wind, the stored thermal energy is used for generating steam to produce electrical energy over a steam turbo generator and the produced electricity is fed in the main supply grid.
A possible solution of thermal energy storage plant is a combination of a charge cycle and a discharge cycle, having in common a thermal energy storage device. The charge cycle comprises, in a closed loop, a fluid transporting machine, e.g. a fan, a heating device, which may be a resistant or inductive heater fed by the electrical power generated by a renewable energy source, and the thermal energy storage device. The discharge cycle comprises, in a closed loop, the same thermal energy storage device of the charge cycle, a fluid transporting machine, e.g. a fan or a blower and a water steam cycle. The water steam cycle includes a thermal machine such as a steam turbine and a heat recovery steam generator (HRSG), a boiler, a heat exchanger or an evaporator, for transferring the thermal energy to water to generate steam which is fed to the thermal machine to produce electrical power from an electrical generator connected to the thermal machine.
The thermal energy storage device comprises typically solid or bulk materials, for example stones, bricks, ceramics and other solid materials, which have high thermal capacity to store thermal energy over a long period of time.
These materials are heated using a working fluid, e.g. air, circulating in the charge cycle, which has a temperature higher than the storage material. In the discharge cycle the stored energy is recovered through a flow of the same or different fluid, which has a lower temperature than the storage material. Due to the temperature differences in flow direction in both cycles, the storage device has a respective hot and a cold end.
In the charge cycle, the thermal energy storage device is connected by a pipe or ducting system to the heating device and to the fluid transporting machine. The fluid transporting machine transports the working fluid through the heating device to the hot end of the thermal storage. A temperature front travels through the storage from the hot end to the cold end. The temperature front is a zone of strong temperature gradient in the storage, which separates the hot and the cold zone in the storage. The charging of the thermal energy storage stops, when the temperature at the cold end begins to rise.
In the discharge cycle the mass flow of the working fluid is guided through the storage device in the opposite direction compared to the charge cycle. In the discharge cycle the working fluid enters the storage at the cold end, reaches the assigned temperature due to heat transfer from the storage material to the working fluid and leaves at the hot end before the working fluid enters the steam generator.
The temperature front travels in reverse direction compared to the charging cycle through the storage device. When discharging the thermal energy storage device the temperature front moves through the storage from the cold end to the hot end. Consequently the temperature of the working fluid which leaves the thermal energy storage device and feeds, for example, the HRSG decreases continuously during a part of the discharging process. This non-homogenous temperature of the working fluid in the discharging circuit leads to a decreasing performance of the thermal machine connected thereto and hence to a non-constant electricity production from the electrical generator.
There may be a need for improving a thermal energy storage plant in such a manner that the above mention inconveniences can be suppressed or reduced in an optimized way.